Flow Control; Throughput Guidelines - US Robotics COURIER User Manual

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COURIER HIGH SPEED MODEMS
remote modem uses a 512- or 1024-entry dictionary. The
size of the dictionary for a call is reported in the A TI6
display.
Bits Entries
9 512
10 1024
11 2048
• Maximum string length of each entry. As the dictionary
fills, the modem deletes the oldest unused strings.
V.42 bis compression is more efficient than MNP5 compression
in part because it dynamically deletes unusable strings. In
addition, it works better with files that are already compressed.
These include .ARC files downloaded from many Bulletin
Boards and 8-bit binary files, which seem to the modem to be
compressed.
MNP5 compression should not be used with such files because it
adds data to them, which lessens throughput. (The additional
data is stripped when the file is decompressed by the remote
modem.) When transferring such files, it's best to set the modem
to &K3: this allows V.42 bis compression to work dynamically
with the compressed data, but disables MNP5.
Flow Control
Flow control of data from the computer is required under error
control for two reasons:
1. The transmitting modem buffers a copy of each frame it
transmits to the remote end until it is acknowledged by the
receiving modem.
2. If errors are encountered, retransmission activity can cause a
steady stream of data from the computer to overflow the
buffer.
Throughput Guidelines
The following guidelines should help to make the most of the
modem's advanced performance features. In many instances,
experimentation and experience will indicate what works best
for your applications.
Link Negotiation and Error Control A-7